This invention relates to frequency shift key demodulator apparatus, and more particularly, to FSK demodulator apparatus which employs digital correlation techniques to enable asynchronous demodulation of FSK signals in a manner consistent with low cost custom semiconductor integration.
Frequency shift key signals result when a carrier is modulated with data by shifting its frequency between two distinct levels, known as mark and space tones or frequencies, based upon the status of the data otherwise transmitted. This form of modulation is frequently employed for purposes of transmission of data over telephone lines or the like, and the decoupling of such data from the transmission medium must be attended by appropriate demodulation at the receiver or within a modem coupling the receiver to the telephone line.
Prior art techniques for the demodulation of FSK signals have generally taken the form of analog filtering approaches, or alternatively, closed loop, phase or frequency-locked decoding techniques have been employed. Each technique results in relatively expensive demodulation equipment which does not readily admit of fabrication through monolithic integrated circuit techniques, and while the same may be adjusted for a high degree of accuracy, they are subject to drift. Hence, demodulators configured in this manner do not manifest a high degree of precision over extended periods of time.
For instance, when analog techniques are employed, the problem of detection of known signals having an unknown phase is approached upon an asynchronous basis employing analog filters and circuitry. Typically the signal to be demodulated is applied to a pair of analog filters, each of which has a passband centered at the mark and space tones of the FSK signal being received. The filters must have a well-defined yet relatively narrow passband and detection is based upon a sensing of the output of each filter. The filter having the largest output is employed for purposes of indicating the presence of the particular mark or space tone associated therewith. Often, the design of such filters is extremely laborious and must be attended by much in the way of signal conditioning circuitry and the like. The resulting demodulator apparatus is expensive to fabricate and does not readily admit of known monolithic integrated circuit techniques on a single substrate. Furthermore, while adjustment of demodulator apparatus of this type can be highly accurate, such apparatus is subject to drift, and hence, precise demodulation of incoming information cannot be reliably achieved over extensive periods of time or through variations in temperature without periodic readjustment of the equipment.
Similarly, when phase or frequency-locked loops are employed for purposes of demodulating FSK information, highly precise closed loop feedback techniques must be employed, and the bandwidth of the phase-locked loop must be generally wider than optimum detection width. This means that FSK demodulators employing phase-locked loop techniques are highly susceptible to interference. Additionally, the resulting analog circuitry is generally difficult to design and costly to implement. These circuits too are not well-suited for implementation through use of conventional monolithic integrated circuit techniques on a single substrate.
Therefore, it is an object of the present invention to provide FSK demodulator apparatus employing open loop digital correlation techniques to achieve highly precise demodulation which is not subject to drift.
A further object of the present invention is to provide FSK demodulator apparatus which is inexpensive to manufacture and readily admits of fabrication as a single substrate monolithic integrated circuit implemented in MOS or bipolar technologies.
Another object of the present invention is to provide FSK demodulator apparatus employing asynchronous, open loop demodulation techniques. An additional object of the present invention is to provide FSK demodulator apparatus which may be readily incorporated within a high speed modem for operation in a dial-up voice telephone network at speeds of up to 1200 baud in a half-duplex mode.
Various other objects and advantages of the present invention will become clear from the following detailed description of several exemplary embodiments thereof and the novel features will be particularly pointed out in conjunction with the claims appended hereto.